1. Field of the Invention
The present invention relates to a motor-driven power steering system adapted to power steer the rear road wheels in combination with a steering motion exerted on the front road wheels, and more particularly, to such a power steering system having a fail-safe mechanism for locking the steering motion of the rear road wheels upon failure of the power steering system.
2. Description of the Prior Art
In a conventional power steering system for front and rear road wheels, there is generally provided a closed loop control in which an appropriate steering angle is directed to the rear road wheels on the basis of the output signals of a steering-wheel sensor and/or a vehicle-speed sensor. The actual steering speed and the actual steering angle of the rear road wheels are then detected and fed back by appropriate means so as to detect a difference between the directed steering angle and the actual steering angle of the rear road wheels. On the basis of this difference, an appropriate level of voltage to be imposed on an electric motor is determined by appropriate arithmetic operations.
The conventional motor-driven power steering system for front and rear road wheels as constructed above has the following disadvantages. Specifically, in order to carry out suitable steering control on the rear road wheels, various sensors such as a vehicle speed sensor, a steering angle sensor, and the like are required. These sensors are mounted at various portions of a vehicle over a relatively broad area and electrically connected with each other or with appropriate control elements through wiring. Accordingly, if one of the sensors has failed or the wiring is cut or broken, undesirable situations, such as uncontrollable steering of the rear road wheels irrespective of the driver's intention, may result. Also, in cases where the electric motor has failed due to breakage or other trouble in the supplying power line, it is hazardous to allow the rear road wheels to move freely irrespective of the steering of the front road wheels.